Effects of Duration of Weed-Free Periods in Organic Lettuce Production.

Weed management accounts for a substantial portion of farm budgets in organic lettuce production. Knowledge of the duration the lettuce crop has to be kept weed-free without compromising yield or quality may result in unnecessary weeding costs. Studies were conducted in 2014 and 2015 to determine the effect of the duration of weed-free period on crop yield and quality of transplanted organic Romaine lettuce.  The crop was grown for 8 weeks, with 8 different weed-free periods [0 (no weed control), 1, 2, 3, 4, 5, 6, 7 (weed-free entire 8 weeks)]. The plots were hand weeded. All standard organic production practices were followed. Data were collected on total and marketable yield, hand weeding costs, weed density, weed biomass, crop quality rating at harvest, and phenolic composition. Results showed that the critical weed-free duration for lettuce yield and quality was up to four weeks after transplant.  However, total stand counts, disease incidence, and phenolic composition of the leaves were not affected by the duration of weed-free period.  The major weed species in the plots depended on the season.  Weed biomass data also showed that there was not much benefit in controlling weeds beyond four weeks after lettuce transplant.  Therefore, it can be concluded that a weed-free duration of four weeks after transplanting will be sufficient to produce quality Romaine lettuce in organic cropping systems with optimum yields and weed control costs.